Compact storage ring extreme ultraviolet free electron laser

ABSTRACT

A high power extreme ultraviolet (EUV) beam is produced. An electron beam is injected in a compact electron storage ring configured for emission of free-electron laser (FEL) radiation. The electron beam is passed through a magnetic undulator on each of a plurality of successive revolutions of the electron beam around the compact electron storage ring. The electron beam is induced to microbunch and radiate coherently while passing through the magnetic undulator. A portion of the free-electron laser radiation at an extreme ultraviolet wavelength produced by an interaction of the electron beam through the magnetic undulator is outputted.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/438,611 entitled COMPACT STORAGE RING EXTREME ULTRAVIOLETFREE ELECTRON LASER filed Feb. 21, 2017 which is incorporated herein byreference for all purposes.

BACKGROUND OF THE INVENTION

The production of upcoming generations of semiconductor circuits for abroad range of applications will require the next generation oflithographic tools which utilize extreme ultraviolet (EUV) lithography.There has been much development using plasma based sources to reach the100 W scale in terms of usable EUV power and efforts are under way toreach 250 W of usable power, but it is unclear if this level can beachieved reliably using the plasma based technology. The low usable EUVpower poses significant challenges in terms of demands forultra-sensitive photoresists leading to shot noise induced roughness andlimitations of high wafer throughput. EUV sources that produce higherpower (e.g., an average power in the range of 1 kW to 3 kW) wouldaddress the current challenges for EUV lithography and provide a viablepath for high volume manufacturing at smaller node sizes, which iscurrently unattainable with plasma-based sources. Thus, there exists aneed for practical high power EUV source.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a diagram illustrating an embodiment of a compact electronstorage ring configured to accept an insertion device suitable for afree-electron laser (FEL).

FIG. 2 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing an undulator magnetinsertion device to create FEL radiation initiated by Self AmplifiedStimulated Emission (SASE).

FIG. 3 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing an undulator magnetinsertion device to create FEL radiation seeded with an externalcoherent source at EUV wavelength or a multiple thereof.

FIG. 4 is a graph illustrating example gain curves for different seedingpower levels in which the FEL output power increases as a function ofposition in an undulator.

FIG. 5 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing a regenerativeself-seeded FEL.

FIGS. 6A-6B are diagrams illustrating embodiments of magnetic undulatorinsertion devices used to produce FEL radiation.

FIG. 7 is a block diagram illustrating an embodiment of a system forperforming EUV lithography.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Producing a high power EUV beam is disclosed. In some embodiments, thesystem includes a compact and/or low-energy electron storage ringconfigured for emission of free electron laser (FEL) radiation. In someembodiments, the compact electron storage ring has a circumference thatis on the order of 30 meters. For example, compact electron storage ringfits inside a 60 square meter area. In some embodiments, the compactelectron storage ring has a circumference that is at least 10 meters butless than 60 meters. In some embodiments, the energy of the electronstorage ring is less than 500 MeV. The use of the compact/low-energyelectron storage ring allows for a significant reduction in the size ofthe system as compared to other high power EUV beam sources. Thereduction in size allows associated financial costs to be reduced andallows the system to be installed in a wider type of environments,including under a floor of a semiconductor fabrication facility. Thesystem further includes an electron injector configured to output anelectron beam into the compact electron storage ring and a magneticundulator (e.g., magnetic undulator insertion device consisting of oneor more discrete undulator magnets) configured to allow the electronbeam to pass through the magnetic undulator. The magnetic undulator mayinclude a plurality of undulators (e.g., the magnetic undulator includesa plurality of different component magnetic undulators). An exitaperture is configured to output at least a portion of the FEL radiationat an EUV wavelength produced by an interaction of the electron beamthrough the magnetic undulator device. Miniaturization of the high powerEUV beam source through the use of the electron storage ring is enabledby various system aspects and inventions. Examples of aspects thatdiffer substantially from prior and standard conventions in the field ofFEL design include: an undulator K parameter that is less than 1, anelectron beam emittance that is greater than λ_(FEL)/4π, operation ofthe EUV FEL in the exponential gain region of an FEL gain curve (i.e.,not in the saturation region), and/or keeping EUV output power below 10%of the FEL saturation power, as described further in the specification.

Free electron laser (FEL) based sources, using various extensions of FELtechniques can be utilized to produce high usable EUV power. FEL schemesuse the property that a magnetic undulator (e.g., periodic structure ofalternating poles of magnets) acts upon a relativistic electron beam toproduce electron micro-bunches that coherently radiate, substantiallyincreasing the radiated power compared to conventional incoherentundulator radiation.

In some embodiments, a self-consistent FEL is utilized rather thanutilizing a “phase-merging enhanced harmonic generation” FEL, a proposalwhere an electron beam is bunched with a “modulator” undulator thenafterwards converted through a dispersive section (e.g., where it caninteract more efficiently with a “radiator” undulator to emit harmonicradiation). For example, much like a klystron rather than true FEL,phase-merging enhanced harmonic generation induces an energy modulationon an electron beam before inducing it to radiate (e.g., using twoseparate undulators). A self-consistent FEL has an insertion device thatprovides a mechanism to both microbunch and coherently radiate withinthe same device (e.g., concurrently in a single undulator magnetdevice).

Typically to generate FEL in prior approaches, a new relativisticelectron pulse is created for each FEL interaction in a linearaccelerator (LINAC) where the electron pulse must be very wellconditioned to produce FEL radiation (e.g., low electron-beam emittanceand energy spread). These FEL sources produce high peak power radiationflashes with a repetition rate of the LINAC and with the sequence ofbunches within each pulse of the LINAC. However, to achieve high averagepower, high repetition rates are required, necessitating asuperconducting LINAC and associated very high capital cost and highpower consumption. Typically these LINACs are physically too large forinstallation in semiconductor fabrication plants for use inphotolithography without significant changes to the construction orlayout of the entire facility.

In some embodiments, instead of implementing the FEL in a lineargeometry following a LINAC, the FEL is incorporated within an electronstorage ring to make efficient use of the electron bunches without theneed to generate and accelerate new electron bunches for every passthrough the FEL undulator. For example, the electron bunches circulatein a storage ring and traverse the FEL undulator periodically andcontinuously without the need to regenerate the electron bunches forevery FEL undulator interaction.

However, the emission of incoherent synchrotron radiation from aconventional weak undulator in a storage ring does not producesufficient power for EUV applications. For example a 330 MeV electronstorage ring with a 7 m long undulator produces only a few watts ofincoherent power in the full spectrum with a single bunch. When this isfiltered down to 2% BW at 13.5 nm, which is required for EUVapplication, the power is less than one Watt average power. Adding morebunches to the storage ring can linearly increase the radiated powerwhich may achieve an average power significantly exceeding one Watt.However, this is far below the necessary power for EUV lithographyapplications. Increasing the magnetic field of the undulator increasesthe incoherent radiated power but also increases the bandwidth of theoutput power so this is not an efficient path to reach high output powerin a narrow spectral bandwidth. On the other hand, the energy storedwithin the electron bunch is very substantial. For example, a 330 MeVelectron bunch with 2.5 nC of charge in a storage ring has on the orderone joule of stored energy. With a repetition rate (revolution frequencyof the storage ring) of order 10 MHz, the average circulating power ison the order of 10 MW. With only 10 bunches, the circulating power is onthe order of 100 MW. Therefore, if the FEL mechanism extracts just0.001% of the power at EUV wavelength, it would yield, as an example, 1kW of EUV power.

Although typical large electron storage rings are hundreds of meters incircumference, in some embodiments, a compact low-energy electronstorage ring is utilized. For example, a compact electron storage ringwith a circumference that is on the order of 30 meters is utilized. Insome embodiments, the compact electron storage ring has a circumferencethat is at least 10 meters but less than 60 meters. In some embodiments,the energy of the electron storage ring is less than 500 MeV. Thecompact electron storage ring implementation achieves a 1 kW to 3 kW (orhigher) average power EUV source, which is more compact, affordable andhas a lower operating cost, than prior art LINAC-based proposals,suitable for application in semiconductor fabrication or otherapplications requiring high average power. In some embodiments, the useof a storage ring substantially increases the interaction rate (to >10MHz) for a circulating pulse to radiate power through multiple passes ina single magnetic undulator by many orders of magnitude.

The utilized storage ring meets the FEL emission conditions for theselected undulator with its equilibrium parameters and the coherentradiation is extracted in such a manner that the electron bunch in thering can be stored in steady-state operation without detrimental changein its equilibrium properties. For example, the storage ring satisfiesthe requirement of storing the electron bunch with a beam quality(emittance) and relative energy spread which are low enough and beamcharge high enough to enable FEL radiation generation in the undulatorboth initially and in the steady state operation during FEL emission. Insome embodiments, the requirements on the emittance of the electron beammay be significantly relaxed compared to LINAC based EUV FELs.

In some embodiments, a compact low-energy electron storage ring iscombined together with a short period undulator (e.g., 1 cm), which isoperated so that it emits FEL radiation at EUV wavelengths (e.g. 13.5nm). The generation of FEL radiation typically has required asufficiently low emittance electron beam with a sufficiently lowrelative-energy spread. In some embodiments, the value required for theemittance may be substantially larger than what was traditionallyrequired for prior approaches. Prior EUV FEL designs operate in thepower saturation regime, which as a consequence increases the relativeenergy spread and degrades substantially the required low emittance ofthe electron beam, thus requiring that the electron bunch be used onlyonce and then discarded or decelerated in a controlled way to recover aportion of its energy and cost. However, when utilizing at least some ofthe embodiments described herein (e.g., an FEL within a compact storagering), this emission may be limited far below saturation tosubstantially preserve the low energy spread and electron beamemittance, but at the same time deep in the amplification regime of theFEL process to harness significantly more power than the incoherentemission from the same undulator. The electron bunch is stored at anequilibrium energy of, for example, 330 MeV. At each circulation of thebeam, it passes through an undulator and emits EUV radiation after anFEL gain of many gain lengths, for example 7 gain lengths, which yieldabout a factor of 1000 gain. The gain length is the physical length ofthe undulator device for which the EUV output power increases by afactor of e (Euler's number) in the exponential gain regime (linear inlin/log plot). The output power is purposely kept below the naturalsaturation power of the FEL. For example, the output may be 10% or lessof the saturation power.

The lost energy is restored by the RF accelerating system of the storagering (e.g., analogous to the restoration of the energy lost due toincoherent radiation). Because the electron bunch is continually storedin the storage ring, there is no other energy lost except that due tothe typical incoherent synchrotron radiation and the additionalincoherent EUV radiation. The beam emittance and energy spread reach anequilibrium state controlled by the natural and induced radiationemission of the storage ring which is restored by the RF acceleratingsystem thereby maintaining a constant energy of the electron beam.

In a free electron laser (FEL) an electron beam is passed through anundulator magnet which causes emission of electromagnetic radiation atthe characteristic wavelength of the undulator, λ_(FEL), given byfollowing equation.

$\lambda_{FEL} = {\frac{\lambda_{u}}{2\gamma^{2}}\left( {1 + \frac{K^{2}}{2}} \right)}$where$K = {\frac{{eB}_{0}\lambda_{u}}{2\pi \; m_{e}c} \cong {0.934{B_{0}\lbrack T\rbrack}{\lambda_{u}\lbrack{cm}\rbrack}}}$

and λ_(u) is the undulator wavelength, γ is the ratio of the electrontotal energy to the electron rest energy, e is the electron charge, B₀is the magnetic field, m_(e) is the mass of the electron and c is thespeed of light. K is the undulator parameter K. The square bracketsenclose the units used for the numerical formula.

A magnetic undulator includes of a series of bending magnets thatalternate in sign with a wavelength of λ_(u), which cause an electronbeam to oscillate in an approximately sinusoidal fashion as it passesbetween the poles. The oscillating electron beam emits synchrotronradiation or undulator radiation incoherently when passing through theundulator. Typical prior undulators for FELs have had been designed withan undulator parameter K that is greater than 1. However, in at leastsome of the described embodiments, an undulator parameter K of less thanone (e.g., substantially less than 1) is utilized.

The gain of a FEL depends upon the parameters of the electron beam andthe undulator. The gain length may be calculated by computer simulationsor by the use of calculations which take into account three dimensionaleffects. In cases where 3D effects start to be important, the gainlength is increased. However, as an example, the one dimensional powergain length L_(G) is given by following equation,

$L_{G} = \frac{\lambda_{u}}{4\pi \sqrt{3}\rho}$

where λ_(u) is the period of the undulator, and ρ is the Pierceparameter.

Prior FEL designs have traditionally imposed various restrictions onparameters, including that: (1) the relative energy spread within thebunch being less than ρ, and (2) the emittance of the bunch being lessthan λ_(FEL)/4π. The restriction of item (1) is important in that theFEL saturation occurs when this condition is satisfied at the end of thegain process. However, this condition may be alleviated by theutilization of the Transverse Gradient Undulator as discussed herein inorder to fulfill the condition locally within the undulator. In this waythe steady state relative energy spread may be significantly larger thanρ. The restriction of item (2) is relaxed in at least some of thedescribed embodiments with proper design of the 3D FEL. A 3D FEL designprocess is utilized in order to arrange the conditions for FEL emission.

Example parameters for a FEL electron storage ring optimized for 13.5 nmoutput is shown in the following Table 1. The parameters have beenspecifically selected so that the storage ring optimization permits theinclusion of an undulator or undulators for the purpose of use with oneor more of the described embodiments. The prior designs typically onlytake into account incoherent emission. In the case of a storage ringwith an integrated FEL, the system provides for additional energy lostby the FEL action by supplying more RF power to the ring. In addition,the system is optimized with the FEL emission as part of the designprocess. For example, the undulator is sufficiently long to providesufficient FEL gain on each pass of the bunch through the undulator(e.g., such gain could be more than 10 but less than 1000). The lengthof the undulator may differ depending upon the embodiment. Gain is alsoinfluenced by the storage ring in that the system produces anequilibrium emittance that is sufficiently small in both transversedegrees of freedom. The system produces a relative energy spread thatmust be of order or less than the FEL ρ parameter. The system producesan equilibrium bunch length which is sufficiently small to provide highpeak current. The system also produces equilibrium transverse emittanceswhich permit FEL radiation. The focusing of the electron bunchthroughout the undulator system is sufficient to provide the necessaryelectron beam density. As shown in Table 1, the emittance issignificantly larger (factor of seven larger than λ_(FEL)/4π) thantypically required in prior systems. Due to the larger emittance, gainlength in the example will be increased somewhat from prior conventionaldesigns, but the increased gain length may be compensated by suitableadjusting the interaction length as part of the design constraint of thecompact storage ring. Additionally, note that undulator parameter K isless than 1 as compared to prior FEL design that require K to be greaterthan 1. The parameters shown in Table 1 are merely illustrative examplesand the parameters in the table may not have been optimized for highaverage power performance of various embodiments.

TABLE 1 Overall Gross Parameters Value Electron Energy, E [MeV] 330.0Circumference, C [m] 23.9 Undulator length [m] 6.5 Undulator period [cm]1.0 Undulator Parameter (K) 0.5 Electron Bunch Charge [nC] 2.0 ElectronBunch length (rms) 2 mm γ* emittance 5 micron Relative energy spread0.0003 1-D FEL ρ Parameter 0.0006 Energy loss per turn, U₀ [keV] 1.9 RFvoltage, V_(RF) [MV] 0.324 RF frequency, f_(RF) [MHz] 1428 Harmonicnumber 114

FIG. 1 is a diagram illustrating an embodiment of a compact electronstorage ring configured to accept an insertion device suitable for aFEL. Electron storage ring 100 is a compact low-energy storage ring. Insome embodiments, electron storage ring 100 has a circumference that ison the order of 30 meters. For example, electron storage ring 100 fitsinside a 60 square meter area. In some embodiments, electron storagering 100 has a circumference that is at least 10 meters but less than 60meters. In some embodiments, the energy of electron storage ring 100 isless than 500 MeV. The shown storage ring 100 includes magneticundulator 202, bending magnets 102, 104, 106, and 108, quadrupolemagnets 110, 112, 114, 116, and 118 for beam focusing, and an RF Cavity120 for replacing energy lost by synchrotron radiation and also to keepthe electrons in tight bunches longitudinally. Not all components havebeen labeled and only a select number of the components have beenlabeled to illustrate the embodiment clearly. Injector 126 generateselectron beams that are bent by bending system 128 (e.g., using one ormore magnets) for insertion in the storage ring. Septum 124 receives theelectron beam and pulsed kicker magnet (kicker) 122 is used to injectthe electron beam into the storage ring. In various embodiments, the useof pulsed kicker magnet 122 is optional.

Injector 126 provides sufficient energy to the electron beam to beinjected and stored in the storage ring. The final energy of theinjector may be either the design energy or a lower energy which issubsequently increased after storage in the storage ring. The injectormay be one of several different types that are familiar to those skilledin the art. In some embodiments, injector 126 includes a linearaccelerator (LINAC). In some embodiments, injector 126 includes a LINACutilized on multiple passes by bending the beam to pass through theLINAC more than once. In some embodiments, injector 126 includes aMicrotron. The emittance of the injected beam is not required to be avery low emittance. The injected emittance is sufficiently low so thatthe beam may circulate enough times in the electron storage ring to cometo equilibrium after cooling to the smaller equilibrium emittancesufficient for FEL emission. The injected energy spread does not have tobe as low as that in the storage ring which is required for FELemission. However, the injected energy spread is sufficient to permitthe electron beam to circulate enough times in the electron storage ringto come to equilibrium after cooling to the smaller equilibrium energyspread sufficient for FEL emission.

In some embodiments, operation of the storage ring FEL begins by theinjection of electrons into the storage ring. In some embodiments,injector 126 creates electron bunches below the energy of the storagering. In this embodiment, the storage ring is ramped in energy afterinjection to the desired final energy, for example 330 MeV in the sampleparameters shown in Table 1. The operation of the EUV may be off fromtime to time for injecting the storage ring and reacceleration to thedesign storage ring energy.

In some embodiments, the storage ring may be injected with an electronbeam with an energy that is equal to that desired for EUV operation. Inthis embodiment, the desired number of bunches may be injected andoperation may be commenced thereafter. From time to time' additionalelectrons may be injected, avoiding the main beam and without disturbingthe operation at EUV, for example after the intensity of the electronbeam is reduced by several percent. This additional beam cools down tobe absorbed into the primary circulating beam. This type of injection issometimes referred as “top up” or trickle charge injection. Thereduction in EUV power may be avoided by utilizing a correspondingincrease in the seed power. This may be controlled by a feedback systemin order to fulfill stability requirements for EUV output power.

The electrons circulate counter clockwise in storage ring 100. Bendingmagnets have a dipole magnet field and may have additional quadrupoleand sextupole fields. Additional magnets (not shown) include quadrupole(e.g., for focusing which keep the electron beam near a stable orbitthat closes after one turn) and sextupole (e.g., used for correcting thechromaticity) magnets. The number of bending magnets 102, 104, 106, and108, and the angular bend of each adds to a total bending of 360degrees. The total number, position and strength of bending magnets,quadrupoles and RF cavities may be optimized to achieve the desiredparameters of the ring including effects due to the emission of FELradiation. The parameters may be optimized including 3D FEL effects sothat, in particular, the emittance may not be less than λ_(FEL)/4π. Thetotal number, position and strength of septums 124 and kickers 122 maybe optimized to achieve the desired injection into the storage ring. Themagnetic field may be selected for each magnet so as to create a stableconfiguration that permits an electron beam to circulate periodically inthe steady state. In some embodiments, storage ring 100 includes asequence of magnets to disperse the electrons laterally according totheir energy before entering an undulator.

Electrons which are stored in the storage ring radiate a significantamount of so called ‘synchrotron radiation’ which also serves to dampthe electrons towards this closed stable orbit, which in turn cools thedistribution of electrons by decreasing the electron beam emittance. Inaddition, because of this constant energy loss, RF cavity 120 isprovided with sufficient power to replace the energy lost due tosynchrotron radiation. This continuous loss and acceleration also servesto damp the electron energy towards the stable periodic orbit, on whichthe amount of radiation is exactly canceled by the acceleration system.This results in the cooling, or decrease, of the relative energy spreadof the beam. In addition to this cooling action, the electron beam isalso heated by the emission of discrete photons. The competition betweenthese effects yield Gaussian distributions in the transverse andlongitudinal directions. Thus, electron storage ring 100 may be used tooptimize the ‘emittance’ of the beam in both the transverse degrees offreedom, as well as the longitudinal degree of freedom (relative energyspread and bunch length). Typical use of electron storage rings havebeen traditionally for the production of incoherent x-rays for researchapplications. However, such prior uses has not been optimized orconsidered for coherent FEL emission at EUV wavelength. Generally theprior designs based on traditional storage rings will not generate veryhigh average power FEL EUV radiation.

The FEL in the storage ring may be operated in various alternative ways.In some embodiments, radiation is initiated by SASE (Self AmplifiedStimulated Emission). In some embodiments, FEL is seeded with anexternal coherent source at 13.5 nm. In some embodiments, FEL is seededwith an external coherent source that has a multiple of the desired EUVwavelength. In some embodiments, FEL is self-seeded by selecting a smallfraction of the output energy of one pulse and then using that energy toseed the next pulse (e.g., regenerative amplifier). The selectedfraction of the output energy may be tuned to reach the desired outputpower.

FIG. 2 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing an undulator magnetinsertion device to create FEL radiation initiated by Self AmplifiedStimulated Emission (SASE). System 200 includes the compact electronstorage 100 of FIG. 1 and magnetic undulator 202. Undulator 202 isadjusted in length so that it is long enough (e.g., 12 gain lengths) forSASE to develop. The length of undulator 202 is also adjusted withselection of electron bunch parameters such that the output power isbelow saturation. The EUV output exiting output aperture 130 includesincoherent undulator radiation and also SASE coherent radiation. Theevolution of the bunching due to the FEL is shown in along theundulator. The beam is shown with one transverse position and thelongitudinal position. Initially the beam is not bunched and iscomprised of a statistical random distribution of particles. Afterprogression along the undulator, the density is starting to be modulatedat the wavelength of FEL emission. Finally at the end of the undulator,the bunching is more extreme, but still not yet fully bunched. The FELis significantly below saturation. After the bunch passes through theundulator it circulates and arrives once again at the beginning. Thebunching is fully washed out by the entrance to the undulator due to thevariation of the path lengths of the particles as they circulate once inthe bending and focusing system.

FIG. 3 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing an undulator magnetinsertion device to create FEL radiation seeded with an externalcoherent source at EUV wavelength or a multiple thereof. System 300includes the compact electron storage 100 of FIG. 1 and magneticundulator 302. FEL amplifies coherent EUV light supplied from EUV seedsystem 304 that generates coherent EUV light. The coherent EUV seedpulse is injected from the right after it is created by the EUV seedsystem 304. After it traverses undulator 302 and interacts with theelectron bunch, it is amplified to emerge on the left and a portion ofthe amplified EUV exits out output aperture 130. The EUV pulse of seedsystem 304 is timed appropriately to be superimposed on the electronbunch as they transverse undulator 302. The length of undulator 302(e.g., about 2 gain lengths plus gain sufficient to reach a sufficientpower for operation, for example 10% of the FEL saturation power) doesnot have to be as long as undulator magnet 202 of FIG. 2 due to theexternally supplied coherent EUV light. A larger seed power results in ashorter undulator length required for power production. For example, ifa power of 1 kW is desired, then a gain of 1000 (e.g., seven gainlengths) is desired, provided that 1 W of coherent EUV average power isused to seed the FEL. In this case, the total gain length required is 9gain lengths which may include 2 gain lengths to form the microbunching(lethargy) and an additional 7 gain lengths to provide a factor of 1000power increase.

FIG. 4 is a graph illustrating example gain curves for different seedingpower levels in which the FEL output power increases as a function ofposition in an undulator. The co-propagating emitted radiation may causebunching of the electron beam at the emission wavelength. This in turncan induce further emission due to the coherent emission of the bunchedbeam. This process can grow exponentially with a characteristic gainlength until the energy lost is approximately equal to the FEL ρparameter, after which the emission saturates. Graph 400 shows exampletypical gain curves for FEL power increase as a function of position inthe undulator. For SASE radiation (e.g., generated using system 200 ofFIG. 2), as shown by curve 402, there are three regions, early gain,exponential (high) gain and finally saturation. For seeded operation(e.g., generated using system 300 of FIG. 3), three curves (i.e., 404,406 and 408) are shown corresponding to different seed power toillustrate the process of seeding. There is an initial delay, calledlethargy (e.g., during which the beam is forming microbunches) beforethe onset of the exponential (high) gain due to the process of bunchingprior to saturation. This delay is about 2 gain lengths, after which thepower grows exponentially according to ex where x is the distancemeasured in gain lengths. The dashed line represents a desired power inthe exponential gain regions of curves 402-408, below the saturation.

In the case of an FEL within a storage ring, the electron beam passesthrough the undulator each time that it circulates around the ring.Therefore, if there is only one bunch of electrons stored in the ringthe pulses of EUV light are produced at the revolution frequency of thestorage ring which is typically of order 10 MHz. In addition, if thereare many bunches in the storage ring, then pulse pattern of emittedradiation follows the pattern of the bunches around the storage ring andrepeats with a frequency of the revolution frequency. If the EUV averagepower desired is 1 kW, then the energy emitted in the form of EUVradiation caused by the FEL action on each turn is 100 micro joulesbased on a revolution frequency of 10 MHz. If there are 10 buncheswithin the ring, then the amount of energy extracted from each bunchwould be correspondingly lower, 10 micro joules for the case consideredabove. For an example ring of the appropriate energy, the typical storedenergy of each bunch is of order 1 joule. Therefore, in the givenexample, the necessary extracted energy is 10 to 100 parts per millionof the energy stored in the ring on each turn.

FIG. 5 is a diagram illustrating an embodiment of a Compact Storage Ringhigh power EUV beam generator system utilizing self-seeded regenerativeamplifier FEL. System 500 includes the compact electron storage 100 ofFIG. 1 and magnetic undulator 502. The length of undulator 502 is longenough to permit amplification of the self-seeded pulse. As shown in theFigure, a portion of the output of the EUV FEL itself is isolated andfed back into the undulator 502 to act as a seed (e.g., instead of usinga separate seed system). The EUV output of system 500 exiting outputaperture 130 includes incoherent undulator radiation and also coherentradiation amplifying the seed pulse. Note that the seed pulse overlapsthe electron beam. To seed the same bunch which created the seed, thetotal delay of the seed is equal to a multiple of the revolution time ofthe electron beam. For example, if the delay is two times the revolutionperiod, then the even revolutions are seeded by even revolutions and theodd revolutions are seeded by odd revolutions (e.g., the firstrevolution seeds the third revolution which seeds the fifth revolutionetc. and similarly with the second revolution, the fourth and the sixth,etc.). Mirrors 504 reflects a portion of the EUV FEL output and thisreflected portion is reflected on mirrors 506, 508 and 510 to be fedback into the storage ring. For example, if the gain is 1000, and if onepercent is circulated back to seed the FEL, then on the next cycle theradiation seed gets amplified by 1000 and the total round trip output isincreased by a factor of 10. Multiple mirrors are utilized to increasethe length of distance traveled by the reflected beam within a compactphysical space. By adjusting the distance traveled by the reflectedbeam, the total time delay of the reflected beam is controlled. In orderfor the seed power to selectively seed the same bunch which created it,the total time delay of the mirrors to provide the EUV seed equals aninteger multiple of the revolution time of the electron bunch. Themirrors shown in FIG. 5 are merely illustrative examples. In variousembodiments, different number of mirrors, mirror geometries and/or othermirror configurations may be utilized.

In the embodiment shown in FIG. 5, to avoid power saturation due toincreasing power with each pass through the undulator magnet, thefraction of radiation power returned for the seeding may be adjusted toa value equal to the inverse of the gain of the FEL to operate the FELin the steady state condition. In some embodiments, a feedback system isutilized to control the output power so that it is well below thesaturation power of the FEL. This may be accomplished by the attenuationor enhancement (e.g., adjust intensity) of the fraction of seed power,which is inserted at the beginning of the FEL (e.g., the intensity ofthe seed is adjusted to maintain the inverse relationship in which thefraction of seed power is equal to the inverse of the total gain of theFEL). Alternatively, properties of the electron beam may be alteredduring operation to modify the amount of gain in the FEL in order tocontrol the output power, so that it is below the saturation power ofthe FEL (e.g., parameters of the electron beam are adjusted to maintainthe inverse relationship in which the fraction of seed power is equal tothe inverse of the total gain of the FEL so as to reach steady state).Alternatively, the properties of the electron beam may automaticallyreach an equilibrium state producing constant output power (e.g., theinverse relationship, in which the fraction of seed power is equal tothe inverse of the total gain of the FEL so as to reach steady state,evolves automatically by evolution of the parameters of the electronbeam). The specific total output power desired may be adjusted by addingor reducing the number of bunches in the storage ring. In the exampleshown in Table 1, the number of bunches may range from 1 to 114.Therefore, the output power could be adjusted over a range of a factorof 114. For example, if the average FEL output power for one bunch were30 W, then the average power could range from 30 W with one bunch to1200 W with 40 bunches to 3420 W with 114 bunches. In some embodiments,the average power may be selected to the appropriate level desired bythe selection of the number of electron bunches stored in the storagering. In some embodiments the bunch to bunch stability may be maintainedby the use of bunch by bunch feedback system.

In some embodiments, with the use of SASE, the output power of the FELmay fluctuate and the coherence of each pulse is independent so that onthe average the illumination will not show coherent effects providedthat it is accomplished on multiple cycles of the storage ring. In someembodiments, with the use of the external seed generation system, thecoherence of the output follows that of the seed. In some embodiments,with the use of self-seeding, the output may develop a single coherentphase, which could lead to undesirable interference effects in the EUVlithography application. Coherent effects in the utilization of the EUVbeam may be altered and eliminated by the utilization of a variation ofthe total length of the seed return path by of order one wavelength attimes scales which are short compared to an illumination time. Forexample, if the illumination time is one second, the path length may beoscillated by more than one wavelength with a frequency of 1 kHz so thatany fringe effects are averaged over 1000 cycles. In some embodiments,length of seed of the external seed generation system may be likewisevaried to alter the phase of the seed pulse. Alternatively, an opticalsystem after the EUV output of the source can be designed to modify beamparameters.

FIGS. 6A-6B are diagrams illustrating embodiments of magnetic undulatorinsertion devices used to produce FEL radiation. FIG. 6A shows a profileview and side view of top and bottom portions of magnetic undulator 602.FIG. 6B shows a profile view and a side view of top and bottom portionsof transverse gradient magnetic undulator 604. A plurality of Magneticundulator 602 or magnetic undulator 604 may be included in any ofmagnetic undulator 202, 302 and/or 502 of FIGS. 2, 3 and 5,respectively. Magnetic undulator 602 creates alternating verticalmagnetic fields that produce a sinusoidal horizontal deflection in theexample shown. The undulator may be oriented to bend vertically orhorizontally. For example, rather than having top and bottom portions ina vertical configuration that bend a beam horizontally, the undulatormay be configured in a horizontal configuration with left and rightportions that bend a beam vertically. The poles may be shaped toinfluence the quadrupole field within the undulator.

The FEL emission has the most significant effect on the energydistribution within the electron bunch. At each wavelength of 13.5 nm,the emission induces a shifted sine wave much smaller that the naturalenergy spread. If this process induces an additional energy spread, eventhough the emission is far below the saturation level, then theequilibrium energy spread of the storage ring may be increased. Forexample, if the natural energy spread is 2×10⁻⁴ and the FEL ρ parameteris 6×10⁻⁴, then FEL emission will take place. However, if this energyspread grows beyond 6×10⁻⁴ as the system reaches equilibrium, then theFEL gain will be reduced on subsequent revolutions of the electron bunchor the FEL may have an equilibrium output from each bunch that isreduced. This effect may be compensated by the use of transversegradient magnetic undulator 604 of FIG. 6B which has a transversegradient in the deflecting field. Undulator 604 has a variation of theundulator parameter transversely caused by tilting the poles of theundulator. This variation can be automatically matched to the increasedenergy spread by having a fixed dispersion of transverse beam positionproportional to the energy within the undulator. In other words,electrons with different energies will automatically enter the undulatorat different transverse positions such that the conditions for coherentemission is maintained along the path through the undulator. Thus, theFEL resonance condition can be maintained in spite of increases in theenergy spread within the electron beam, even at steady-state.

The tipping angle of the poles produces a linear dependence of thevertical bending field with the horizontal position x. The change of thefield with x depends upon the choice of angle. The poles may be alsoshaped to influence the quadrupole field in the undulator. In a storagering the dispersion of the electron beam with energy spread iscontrolled by the detailed design of the magnet lattice, the sequenceand strength of bending and quadrupole focusing magnets. This latticemay be adjusted to provide a dispersion in position with energy spreadwithin the undulator. Alternatively, additional magnets may be used bothupstream and downstream of the undulator to provide a dispersion inposition with energy spread within the undulator. In either case, thismay be used to achieve a matching of the undulator condition of thepreviously discussed λ_(FEL), equation, locally in the disperseddirection. Thus, the equilibrium energy spread may be allowed to exceedthe ρ parameter while still achieving EUV FEL emission.

In some embodiments, the undulator magnet insertion device includes asequence of undulator sections with either spaces or magnets in betweenthem. The spacing of these separate undulator sections may be adjustedso that the FEL action is undisturbed. The spacing of these separateundulator sections may be adjusted to affect the other parameters of thebeam so as to improve the performance of the FEL as operated within thestorage ring. Such spacing may diminish the output of the FEL in asingle pass while improving the overall performance of the system in thesteady state. For example, a shift of FEL phase may be included betweenundulator sections such that the effect of the FEL on the steady stateelectron beam parameters (e.g., relative energy spread) is decreased ordiminished. While this may or may not lead to decreased power emitted bythat bunch of the FEL, the overall performance may be enhanced ormaintained by increasing the number of bunches or the overall current inthe storage ring.

In some embodiments, the electron beam, including a sequence of electronbunches, is steered within the undulator in order to inhibit theemission of EUV FEL radiation temporarily on a sequence of revolutions.For example, such a beam steering may be included such that the effectof the FEL on the steady state electron beam parameters (e.g. relativeenergy spread) is decreased. For example, the beam is steered on 90% ofthe revolutions in order to inhibit the emission of FEL radiation, whileon the remaining 10% the beam is not steered so that the FEL emissionwill occur. In this example, to achieve the same output power, the gainof the FEL may be increased by a factor of 10 on those passes that arenot steered. During those revolutions where the beam is steered and theFEL emission is suppressed, the electron beam continues to be cooledtowards equilibrium values. Such cooling may be advantageous incontrolling the steady state values of beam parameters (e.g. therelative energy spread).

FIG. 7 is a block diagram illustrating an embodiment of a system forperforming EUV lithography. EUV source 702 has been installed on asubfloor under a floor of a semiconductor manufacturing facility cleanroom. The compact size EUV source 702 enabled by the use of a compactstorage ring (e.g., compact storage ring 100 of FIG. 1) has allowed EUVsource 702 to be small enough to fit within a typically sizedsemiconductor manufacturing facility. Examples of EUV source included inEUV source 702 include system 200, system 300 or system 500 of FIGS. 2,3 and 5. The EUV beam output generated by EUV source 702 is reflected bymirror 704 up to EUV optics 706 of EUV lithography system/stepper 708(e.g., lithography scanner) for use as the light source of EUVlithography. In some embodiments, the same beam output generated by EUVsource 702 is provided a plurality of lithography steppers/scanners.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system for producing a high power extremeultraviolet (EUV) beam, including: a compact electron storage ringconfigured for emission of free-electron laser (FEL) radiation; anelectron injector configured to insert an electron beam into the compactelectron storage ring; a plurality of bending magnets and a plurality ofquadrupole magnets interspersed along the compact electron storage ring,wherein at least a corresponding one of the quadrupole magnets isbetween any two of the bending magnets along the compact electronstorage ring; a magnetic undulator configured to allow the electron beamto pass through the magnetic undulator where the electron beam isinduced to microbunch and radiate coherently; and an exit apertureconfigured to output a portion of the free-electron laser radiation atan extreme ultraviolet wavelength produced by an interaction of theelectron beam through the magnetic undulator, wherein the output portionof the free-electron laser radiation is provided to a lithography systemas a light source for the lithography system and an average power of theoutput portion of the free-electron laser radiation is greater than 250W.
 2. The system of claim 1, wherein the magnetic undulator is atransverse gradient undulator that includes a plurality of transversegradient undulator components and each of the transverse gradientundulator components includes a periodic structure of mechanicallycoupled set of magnetic components with alternating poles that createalternating transverse magnetic fields along at least a portion of alength of the transverse gradient undulator to generate at least aportion of the free-electron laser radiation when the electron beamtravels along at least the portion of the length of the transversegradient undulator, and for each of the plurality of transverse gradientundulator components, the included corresponding mechanically coupledset of magnetic components with alternating poles is uniformly tilted ina transverse direction to a path of the electron beam.
 3. The system ofclaim 1, wherein an undulator parameter K of the magnetic undulator isless than
 1. 4. The system of claim 1, wherein an electron beamemittance (ε) of the system is greater than λ_(FEL)/4π.
 5. The system ofclaim 1, wherein an equilibrium relative energy spread is greater than aFEL ρ parameter of the system.
 6. The system of claim 1, wherein foreach of the plurality of transverse gradient undulator components, thecorresponding transverse gradient undulator component is uniformlytilted for the entire length of the corresponding transverse gradientundulator component.
 7. The system of claim 1, wherein a spacing betweensections of the magnetic undulator is configured to diminish an impactof an FEL action on electron beam parameters.
 8. The system of claim 1further comprising, one or more magnets configured to laterally disperseelectrons of the electron beam according its energy before entering themagnetic undulator.
 9. The system of claim 1, wherein EUV FEL of thesystem is operated in an exponential gain region of a FEL gain curve.10. The system of claim 1, wherein the electron beam is in a steadystate.
 11. The system of claim 10, wherein the steady state is reachedbetween a cooling of the electron beam in the compact storage ring and aheating of the electron beam due to incoherent and FEL processes. 12.The system of claim 1, wherein FEL radiation is initiated via SelfAmplified Stimulated Emission.
 13. The system of claim 1, wherein FELradiation is initiated using an external separate coherent EUV source.14. The system of claim 1, wherein a portion of the output FEL radiationis seeded back to the compact electron storage ring.
 15. The system ofclaim 14, wherein the portion of the output FEL radiation seeded back tothe compact electron storage ring is delayed by a multiple of arevolution time of the electron beam around the compact electron storagering.
 16. The system of claim 14, wherein based on an inverse of a totalgain of the FEL radiation, the seeded portion of the output FELradiation is selected to reach a steady state.
 17. The system of claim16, wherein a power, an intensity or a parameter of the seeded portionof the output FEL radiation is selected or adjusted to maintain theinverse relationship of the total gain of the FEL radiation required toreach the steady state.
 18. The system of claim 1, wherein a number ofelectron bunches stored in the compact storage ring is adjusted and setto achieve a desired total output power.
 19. The system of claim 1,wherein for a portion of a plurality of times the electron beam passesthrough the magnetic undulator, the electron beam is steered within themagnetic undulator to reduce the emission of the FEL radiation duringthe portion of the plurality of times the electron beam passes throughthe magnetic undulator.
 20. The system of claim 1, wherein the outputportion of the FEL radiation is provided to the lithography system via amirror.
 21. The system of claim 1, wherein the system is installed in asemiconductor fabrication is facility.
 22. A method for producing a highpower extreme ultraviolet (EUV) beam, including: injecting an electronbeam in a compact electron storage ring configured for emission offree-electron laser (FEL) radiation, and the compact electron storagering includes a plurality of bending magnets and a plurality ofquadrupole magnets interspersed along the compact electron storage ring,and at least a corresponding one of the quadrupole magnets is betweenany two of the bending magnets along the compact electron storage ring;passing the electron beam through a magnetic undulator on each of aplurality of successive revolutions of the electron beam around thecompact electron storage ring, wherein the electron beam is induced tomicrobunch and radiate coherently while passing through the magneticundulator, and the magnetic undulator is a transverse gradient undulatorthat includes a plurality of transverse gradient undulator componentsand each of the transverse gradient undulator components includes aperiodic structure of mechanically coupled set of magnetic componentswith alternating poles that create alternating transverse magneticfields along at least a portion of a length of the transverse gradientundulator to generate at least a portion of the free-electron laserradiation when the electron beam travels along at least the portion ofthe length of the transverse gradient undulator, and for each of theplurality of transverse gradient undulator components, the includedcorresponding mechanically coupled set of magnetic components withalternating poles is uniformly tilted in a transverse direction to apath of the electron beam; and outputting a portion of the free-electronlaser radiation at an extreme ultraviolet wavelength produced by aninteraction of the electron beam through the magnetic undulator, whereinthe output portion of the free-electron laser radiation is provided to alithography system as a light source for the lithography system and anaverage power of the output portion of the free-electron laser radiationis greater than 250 W.
 23. The method of claim 22, wherein the magneticundulator includes a transverse gradient undulator.